This invention relates to the treatment of effluent waste streams normally discharged by tanneries that manufacture sole, harness, saddle, and other leathers using vegetable extract tannins. Tanneries engaged in the production of leather are based upon chrome tannage, vegetable extract tannage, or a combination of both. In combination tannage, the extract or its equivalent is used as a retanning agent after chrome tanning has taken place. Sometimes the sequence is reversed.
Most of the waste streams that are discharged by vegetable extract tannaries are quite similar to the ones discharged by chrome tanneries. Soaking, washing, dehairing, (hair pulping or hair saving), liming and reliming, bating, pickling, fat liquoring, coloring and finishing introduce lime, sulfides, amines, proteins, soluble salts of ammonia, sodium and calcium, emulsified fats, oils and waxes, traces of dyes, pigments, and coloring materials into the final waste streams of both vegetable tanneries and chrome tanneries. All of these streams except the final pickling and tanning steps are predominantly alkaline, and the waste streams from dehairing and liming are strongly alkaline. Therefore, the composite waste effluent is predominantly alkaline. conventional procedures that are used for treatment of such waste streams include:
(1) preliminary screening or sieving of tannery wastes to remove hair, flesh strings, and particulate solids; PA1 (2) primary settling to remove floatables by skimming and remove settleable solids as sludge; PA1 (3) equalization of streams to permit the treatment of a more or less uniform effluent flow; PA1 (4) use of manganese salts to catalytically oxidize sulfides by aeration at the ambient high pH with simultaneous displacement of free ammonia; PA1 (5) a second clarification to remove grease released by aeration and remove additional precipitated solids; PA1 (6) the use of flocculants to aid and abet the coalescence and settling of precipitated solids; and PA1 (7) neutralization of the final primary effluent by the incorporation of acidic substances. PA1 a. Drippage and draining of vegetable tanned stock. PA1 b. Drying of vegetable tanned leather promotes surface oxidation which in many cases produce very dark colored areas. This requires a subsequent alkaline wash to redissolve the objectionable colorations followed by an "acid bleach" from which operations, the waste streams contain further quantities of the troublesome "red waters". PA1 c. When spent extract liquors from any operation contain insufficient tannin to justify concentration and recovery, they too find their way into the waste streams to further plague the primary treatment of tannery effluents.
Notwithstanding the similarities in many of the waste streams found in chrome and vegetable tanneries, certain problems arise in the primary treatment of vegetable tanning waste streams that are not encountered in chrome tannage operations. The conventional treatment methods are not entirely adequate to deal with these special problems, which arise due to the presence of soluble vegetable tannin materials in the waste effluent.
Numerous extracts are commonly used in vegetable tanneries and are prepared from a number of tree barks or the wood itself. Among the more important of these are quebracho, chestnut, mangrove, wattle, spruce, oak, hemlock, etc. Certain nuts or pods such as myrobalans, valonia, and divi-divi have also been processed. All of these vegetable substances yield tannins in greater or lesser amounts when extracted with boiling or hot water. These tannin solutions are then used in vegetable tanneries to convert a prepared hide or skin into leather using numerous steps and operations that comprise the tanner's art.
Vegetable tannins have little color at the lower pH values (pH about 3.0) which prevail under the initial conditions of their use. Under these conditions, their color may range from pink to light brown. At the prevalent pH of a final composite tannery waste effluent (pH about 11-12), however, the vegetable tannins are readily oxidized, and become deep red or even very deep red-violet. It is necessary to remove substantially all of these color bodies in order to produce an effluent that will not produce deep colorations when the effluent enters the receiving stream. This necessity poses a significant problem, because it is virtually impossible to keep the vegetable tannin materials from entering the waste effluent, secondary "activated sludge" treatment is not effective to remove the color bodies, and removal of the bodies by primary treatment methods has been a slow and inefficient process.
As in many other industrial chemical processes, particularly those in which chemicals are absorbed by a substrate, there is rarely a complete utilization of the absorbed reagents whether it be tannin or some other chemical substance. As a result, recycling is a common practice so as to avoid waste, and this is done so long as excessive dilution does not render the cost prohibitive, but an inevitable quantity is discharged. The following operations for example result in introduction of the vegetable tannin materials into the final waste effluent:
When such discharges take place, the extract material reacts with numerous other tannery waste streams to produce soluble color bodies, which must then be removed.
Even though excess vegetable extracts are precipitated by increased pH values due particularly to lime or magnesia, the precipitated particles do not settle rapidly enough to be removed through normal clarifier operations. In fact, precipitation of "reds" by lime under alkaline conditions requires many days or weeks of settling before an effluent of suitable color can be discharged to the receiving waterway. Such a process requires many acres of sludge lagoons.
The use of many varieties of flocculants has not been effective in speeding up the sedimentation step to any appreciable degree. Normal use of anionic polyacrylamide type flocculants does not promote satisfactory coalescence of the precipitate. Simultaneous use of iron salts as primary flocculants produces deep blue-black inks when the vegetable tannins are present. Aluminum and chromium salts, while producing no discoloration, are not very effective. Vegetable tannins react with the precipitate proteinaceous materials, but these dispersed solids are again too finely divided to settle out, and they do not lend themselves to efficient flocculation.
A principal object of the present invention is to provide a process for treating the waste effluent of vegetable tanning operations that will quickly and efficiently remove substantially all of the deep red color bodies.
Another object of the invention is to provide such a process which will efficiently remove other suspended solids from tannery waste effluents. Another object is to provide such a process in which iron salts may be used as flocculants without producing a final effluent that has dark blue-black colorations.